I didn't say it was. States run elections and have their own laws and procedures, yes, even for federal positions providing they do not violate the federal Constitution which says only that candidates for president must be natural born citizens and 35 or older. The 14th Amendment does state that no one is eligible to serve as president if they engaged in insurrection against the US government, but, oh well, only Congress can enforce that and we know it will not.
A state does determine who can vote under their state's laws provided the various voting rights acts are not violated and that no one's personal Constitutional rights are violated. Some states disenfranchise felons while others do not.
Disenfranchisement laws exclude millions of Americans from participating in our democracy. The Brennan Center tracks these laws state by state.
www.brennancenter.org
This is an argument of States' Rights vs Federal. Currently, there is no such thing as a federal voter id.